Electron emissive composition and method of application



July 13, 1954 2,683,837

J. A. HAGER ELECTRON EMISSIVE COMPOSITION AND METHOD OF APPLICATION Filed Jan. 12, 1951 lnvervL-ov: James Aha q er', 8 Wf- His A tiovneg.

Patented July 13, 1954 [TED STATE ELECTRON EMISSIVE COMPOSITION AND METHOD OF APPLICATION James A. Hager, Bucyrus, Ohio, assignor to General Electric Company, a corporation of New York Claims.

My invention relates to electric discharge devices having at least one activated cathode, and more particularly to the manufacture of such devices and to compositions of electron emissive coatin s for the cathodes.

It is an object of my invention to improve the adherence of electron emissive coatings to cathdes. It is a further object to provide a method of manufacturing such devices wherein adherence of the activating coating is assured during manufacture of the device. It is still another object to provide a novel coating composition which will assure retention of the activating composi -on in a cathode during fabrication of device.

In the usual process of manufacturing electric discharge devices such as fluorescent lamps the cathode, usually a coiled coil of tungsten Wire, is carried by support or lead-in wires sealed in a glass stem, the assembly being termed a mount. The cathode coil is coated, as by dipping in a susof electron emissive material usually comprising alkaline earth carbonates including calcium and strontium carbonates which be used singly or as double or triple cariates and which are carried in suspension in a cellulosic binder such as a solution of nitrocellulose in a suitable solvent like amyl acetate. m unt is then sealed by fusion in a glass env pe which is subsequently evacuated, the cathode being heated to volatilize the binder and convert the alkaline earth carbonate to oxide, and the envelope being filled with a suitable ionizable atmosphere such as one or more inert s like neon, argon, krypton, etc. and mercury.

t been noted that in some instances the devices processed in the manner described above failed in life. I have found that the early failures were due to a deficiency, or even a total absence, of activating or emissive material on the cathodes of the finished lamps. More particul' have found that the loss of emissive materi 1 occurred during the interval between the toes of sealing the cathode into the envelope and evacuating the envelope, the loss being ocby the fact that during the sealing-in eration the cathode was subjected to a temature above the volatilization temperature of l the cellulosic binder in the activating coating with the result that the binder was lost and the alzialine earth compounds remained as a dry powder which was then thrown out of the oathode by bumping or jarring the lamp in the normal course of handling it thereafter and before the coating had been sintered to the cathode coil during activation on the exhaust pump.

In accordance with the present invention, the problem of loss of emission material is overcome by the addition to a conventional emission mixture including alkaline earth compounds suspended in a solution of a cellulosic binder, of additive or secondary binder consisting of a granular resin, preferably a thermo-setting resin which is cured to a solid plastic during the aforesaid sealing-in operation so as to firmly bond the emissive material to the cathode and maintain the bond through the sealing-in operation and until the emissive material has been sintered to the cathode during activation in conjunction with the subsequent exhausting of the device containing the cathode.

It has been determined that during the normal sealing-in operation the cathode reaches a temperature of about 300380 G. Since the binder, nitrocellulose for example, decomposes at about 209 C., it is completely burned out on sealing. The additive, or secondary binder, should therefore be one which is cured to a solid plastic at a. temperature below 300 C. and, preferably, even below the decomposition temperature of the cellulosic primary binder, but which will withstand a temperature of about 400 C. during sealing and will burn out at some temperature below the activation temperature of about 1200 C. on the exhaust machine. I have found that particularly good results are obtained by employing as additive a phenolic resin, such as a phenol-furfural resin, particularly that known as Durite Resin Powder No. 275 which is a mixture of a condensation product of phenol and furfural with hexamethylenetetramine and is supplied by the Chemical Division of The Borden Company. This Durite cures to a solid plastic at about 150-169" C. in a very short time ('70 seconds) and decomposes at about 45o- 500 C. Therefore, when added to the emission mixture it becomes solid during the sealing-in operation, just before the nitrocellulose primary binder starts to oxidize, and binds the emission material to the cathode.

It will be seen that I provide an emissive mixture employing two diiierent binders each of which fulfills a different function in the coating process. The cellulosic primary binder possesses the proper viscosity to assure proper filling of the coils of the cathode and firmly retains the emissive mixture therein at ordinary temperatures whereas the resinous secondary binder (the presence of which does not increase the viscosity of the primary binder) takes over the binder function when temperatures are reached sufficient to. volatilize or decompose the cellulosic binder. The Durilte binderis used in small amounts and very little, if any, remains in the finished lamp. It breaks down into carbon, oxygen, hydrogen and water. The latter three materials are pumped out and some of the carbon is probably oxidized and pumped out. The remaining amount of carbon is so small that it approaches the quantity left after the carbonates of the 6.1 ission mixture break up.

Suitable cellulosic primary binders include esters and others such as nitrocellulose, ethyl cellulose, cellulose acetate, etc. dissoved in suitable solvents such as amyl or butyl acetate, Cellosolve acetate, etc. It should be noted that the Durite, in time, affects the stability of nitrocellulose binder and reduces its viscosity, so that it should be added to the emission mixture just prior to use and should be used the same day. Ethyl cellulose is not so affected. However, I prefer to employ nitrocellulose inasmuch as it more readily provides a complete filling of the cathode coil.

In general, I prefer to use the minimum amount of the secondary binder which will assure good adherence. Good results are obtained with small amounts of the Durite secondary binder, for example 2 to grams per 180 ml. of a conventional suspension of emission mixture consisting of about 130 to 220 grams of alkaline earth carbonates in about 130 to 140 ml. of a nitrocellulose binder having a viscosity of about to centipoises.

If desired, the emission mixture may be further modified by the addition thereto of a small amount, about 3 to 5 grams, of finely powdered metallic zirconium.

For a further understanding of the invention reference may be had to the accompanying drawing illustrating certain steps in the manufacture of a discharge device such as a fluorescent lamp,

and wherein Fig. l is an elevation of a mount structure carrying a coiled cathode, Fig. 2 is an elevation illustrating the step of sealing the mount into the lamp envelope, and Fig. 3 is an elevation illustrating the exhaust operation.

Referring to Fig. 1, the mount comprises a glass stem l through which extend a pair of lead-in wires 13 carrying a cathode coil 3 which may be a coiled coil of tungsten wire. This cathode coil 8 is filled or coated with an emission mixture of the character described above comprising alkaline earth carbonates suspended in a solution of a cellulosic binder and containing an additive of a resinous binder material.

As shown in Fig. 2, the mount is sealed into a tubular glass envelope l by fusion of the flared end of the stem 5 to the end of the envelope 6. In the conventional fluorescent lamp the envelope 4 is coated interiorly with a fluorescent powder and a mount of the type shown in Fig. 2 is sealed to each end of the envelope. As pointed out above, the sealing fires 5 cause the cathode to be heated to a temperature of the order of file-380 C. so that the cellulosic binder in the emission mixture is burned out and the resinous secondary binder is cured to a solid plastic which firmly binds the emission coating to the cathode 3 throughout the sealing-in operation and throughout normal handling of the lamp until activation of the emission mixture on the exhaust machine.

Fig. 3 illustrates the exhaust operation wherein the envelope 4 is heated and is evacuated through an emaust tube 6 extending from the stem 1 at the upper end of the envelope. During this exhaust operation the cathodes 3 and and 3' are heated, preferably by passage of electric current therethrough, to a temperature of approximately 1200 C. to break down the secondary resinous binder and to convert the alkaline earth carbonates of the emission mixture to the corresponding oxides. As a result of this process the emissive coating is sintered and firmly bonded to the cathode so that there is no longer any danger of its being dislodged from the cathode upon bumping or jarring the lamp.

While I have described certain aspects of my invention in detail, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In the manufacture of an electric discharge device comprising a sealed envelope containing an activated cathode, the method which comprises preparing an activating mixture comprising a suspension of finely divided particles of alkaline earth metal carbonate in a primary cellulosic binder and adding thereto a secondary binder of granular thermo-setting resinous composition having a volatilization temperature materially above that of said cellulosic binder and in proportions corresponding approximately to 2 to 5 grams of the said resinous composition per m1. of suspension comprising about 130 to 220 grams of alkaline earth carbonate in about 130 to ml. of cellulosic binder solution having a viscosity of about 30 to 100 centipoises, applying the said activating mixture to said cathode, sealing the cathode into the envelope by application of heat sufficient to volatilize the said primary cellulosic binder and to concomitantly cure the said secondary resinous composition to a solid plastic which retains the said carbonate in place on the cathode, and subsequently evacuating the envelope and heating the cathode to a temperature sufilcient to decompose the alkaline earth compound and to volatilize the said resinous composition.

2. In the manufacture of an electric discharge device comprising a sealed envelope containing an activated filamentary coiled cathode, the method which comprises preparing an activating mixture comprising a suspension of finely divided particles of alkaline earth metal carbonate in a primary cellulosic binder solution of proper viscosity to adequately fill the coils of said cathode and adding thereto a small amount of a secondary binder of thermosetting resin having a volatilization temperature materially above that of the cellulosic binder, coating the cathode with the said activating mixture, sealing the coated cathode into the envelope by application of heat sumcient to volatilize the primary cellulosic binder and to cure the secondary resin binder to a solid plastic which retains the said carbonates in place on the cathode, and subsequently evacuating the envelope and heating the cathode to a temperature sufficient to volatilize the said secondary resin binder and to decompose the said carbonate.

3. In the manufacture of an electric discharge device comprising a sealed envelope containing an activated filamentary coiled cathode, the method which comprises preparing an activating mixture comprising a suspension of finely divided particlesof alkaline earth metal carbonate in a primary binder of nitrocellulose solution of proper viscosity to adequately fill the coils of said cathode and adding thereto a secondary binder of thermo-setting resin of the phenolic condensation type having a volatilization temperature materially above that of the nitrocellulose binder and in an amount sufficiently small that it does not materially lower the viscosity of the nitrocellulose solution over an appreciable period of time, coating the cathode with the said activating mixture before the viscosity of the nitrocellulose solution has been materially reduced by the presence of the resin, sealing the coated cathode into the envelope by application of heat sufii'cient to volatilize the primary nitrocellulose binder and to cure the secondary resin binder to a solid plastic which retains the said carbonates in place on the cathode, and subsequently evacuating the envelope and heating the cathode to a temperature suilicient to volatilize the said secondary resin binder and to decompose the said carbonate.

4. In the manufacture of an electric discharge device comprising a sealed envelope containing an activated filamentary coiled cathode, the

method which comprises preparing an activating mixture comprising a suspension of finely divided particles of alkaline earth metal carbonate in a primary cellulosic binder solution of proper viscosity to adequately fill the coils of said cathode and adding thereto a small amount of a secondary binder of thermosetting phenol-furfural resin having a volatilization temperature materially above that of the cellulosic binder, coating the cathode with the said activating mixture, sealing the coated cathode into the envelope by application of heat suflicient to volatilize the primary cellulosic binder and to cure the secondary resin binder to a solid plastic which retains the said carbonates in place on the cathode, and subsequently evacuating the envelope and heating the cathode to a temperature sufiicient to volatilize the said secondary resin binder and to decompose the said carbonate.

5. In the manufacture of an electric discharge device comprising a sealed envelope containing an activated filamentary coiled cathode, the method which comprises preparing an activating mixture comprising a suspension of finely divided particles of alkaline earth metal carbonate in a primary binder of nitrocellulose solution and adding thereto a secondary binder of thermo-setting resin comprising the condensation product of phenol and furfural with hexamethylenetetram-ine and having a volatilization temperature materially above that of the nitrocellulose binder and in proportions corresponding approximately to 2 to 5 grams of the said resin per 180 ml. of suspension comprising about 130 to 220 grams of alkaline earth carbonate in about 13o to 140 ml. of nitrocellulose binder solution having a viscosity of about 30 to 109 centipoises, coating the cathode with the said activating mixture before the viscosity of the nitrocellulose solution has been materially reduced by the presence of the resin, sealing the coated cathode into the envelope by application of heat sumcient to volatilize the primary nitrocellulose binder and to cure the secondary resin binder to a solid plastic which retains the said carbonates in place on the cathode, and subsequently evacuating the envelope and heating the cathode to a temperature sufilcient to volatilize the said secondary resin binder and to decompose the said carbonate.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,691,446 Rentschler Nov. 13, 1928 1,795,918 Wilson Mar. 10, 1931 2,310,866 Pitman Feb. 9, 1943 2,363,028 Warnke Nov. 21, 1944 

1. IN THE MANUFACTURE OF AN ELECTRIC DISCHARGE DEVICE COMPRISING A SEALED ENVELOPE CONTAINING AN ACTIVATED CATHODE, THE METHOD WHICH COMPRISES PREPARING AN ACTIVATING MIXTURE COMPRISING A SUSPENSION OF FINELY DIVIDED PARTICLES OF ALKALINE EARTH METAL CARBONATE IN A PRIMARY CELLULOSIC BINDER AND ADDING THERETO A SECONDARY BINDER OF GRANULAR THERMO-SETTING RESINOUS COMPOSITION HAVING A VOLATILIZATION TEMPERATURE MATERIALLY ABOVE THAT OT SAID CELLULOSIC BINDER AND IN PROPORTIONS CORRESPONDING APPROXIMATELY TO 2 TO 5 GRAMS OF THE SAID RESINOUS COMPOSITION PER 180 ML. OF SUSPENSION COMPRISING ABOUT 130 TO 220 GRAMS OF ALKALINE EARTH CARBONATE IN ABOUT 130 TO 140 ML. OF CELLULOSIC BINDER SOLUTION HAVING A VISCOSITY OF ABOUT 30 TO 100 CENTIPOISES, APPLYING THE SAID ACTIVATING MIXTURE TO SAID CATHODE, SEALING THE CATHODE INTO THE ENVELOPE BY APPLICATION OF HEAT SUFFICIENT TO VOLATILIZE THE SAID PRIMARY CELLULOSIC BINDER AND TO CONCOMITANTLY CURE THE SAID SECONDARY COMPOSITION TO A SOLID PLASTIC WHICH RETAINS THE SAID CARBONATE IN PLACE 